1. Field of the Invention
The present invention relates to multiple access communication systems, such as wireless data or telephone systems, and satellite repeater type spread spectrum communication systems. More particularly, the invention relates to a method and apparatus for acquiring and tracking digital signals being transmitted within a communication system using energy from multiple Walsh function encoded channels. The invention further relates to a method of acquiring pilot signals in code division multiple access spread spectrum type communication systems.
2. Description of the Related Art
A variety of multiple access communication systems have been developed for transferring information among a large number of system users. Two known techniques employed by such multiple access communication systems include time division multiple access (TDMA) and frequency division multiple access (FDMA), the basics of which are well known in the art. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over the other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in a multiple access communication system is disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", and U.S. patent application Ser. No. 08/368,570, filed under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," which are both assigned to the assignee of the present invention, and incorporated herein by reference.
These patents disclose multiple access communication systems in which a large number of generally mobile or remote system users or subscribers each employ at least one transceiver to communicate with other system users or desired signal recipients, such as through a public telephone switching network. The transceivers communicate through gateways and satellite repeaters, or terrestrial base stations (also sometimes referred to as cell-sites or cells) using code division multiple access (CDMA) spread spectrum type communication signals. Base stations cover cells which are defined by the effective `reach` of their signals, while satellites form beams which cover a `spot` produced by projecting satellite communication signals onto the Earth's surface. In addition, cells are generally divided into different geographical regions referred to as sectors, while satellite beams at different frequencies, sometimes referred to as FDMA signals, beams, or sub-beams, may cover a common geographical region. The geographic regions serviced are similar in nature differing in the physical characteristics of the type of repeater platform used and its location. Although, certain characteristics of the transmission paths and restraints on frequency and channel reuse may also differ between these platforms.
In CDMA communication systems, a set or pair of preselected pseudorandom noise (PN) code sequences are used to modulate or `spread` user information signals prior to modulation onto a carrier signal for transmission as communication signals. In the base station- or gateway-to-subscriber communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams from gateways, as well as between multipath signals. These codes are typically shared by all communication signals within a cell or beam. Channelizing codes are used to discriminate between different users within a cell or between user signals transmitted within a satellite beam on a forward link. That is, each subscriber unit has its own orthogonal channel provided on the forward link by using a unique `covering` orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical length being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems. In this arrangement, each Walsh function of 64 or 128 chips is typically referred to as a Walsh symbol.
In addition, some signal diversity is used as one approach to reduce the deleterious effects of fading and additional problems associated with relative user, or satellite repeater, movement within the communication system. Generally, three types of diversity are used in spread spectrum communication systems, and they are time, frequency, and space diversity. Time diversity is obtainable using repetition and time interleaving of signal components. A form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space diversity is provided using multiple signal paths, typically, through different antennas or communication signal beams.
Typical CDMA spread spectrum communication systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation for forward link subscriber unit communications. In communication systems using this approach, a `pilot` carrier signal is used as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station or gateway throughout a region of coverage. A single pilot is typically transmitted by each gateway or base station for each frequency used. This pilot is shared by all users receiving signals from that source. Generally, sectors each have their own distinct pilot signals while satellite systems transfer a pilot within each satellite beam frequency, or sub-beam, which originates with gateways using the satellite.
Since pilot signals do not typically involve data modulation, they essentially consist of PN spreading codes which are modulated onto a carrier frequency. Pilot signals use the same PN spreading code or set of codes throughout the communication system but with different relative code timing offsets for each beam, cell, or sector. This provides signals that can be readily distinguished from each other, also distinguishing between beams and cells while providing simplified acquisition and tracking. Other signals are used to transmit spread spectrum modulated information, such as gateway or base station identification, system timing, user paging information, and various other control signals.
Pilot signals are used by subscriber units to obtain initial system synchronization, and time, frequency, and phase tracking of transmitted signals. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of other system or user information signals. This technique allows many user signal carriers to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism.
As part of the process of establishing a communication link, the subscriber unit transceiver employs a receiver referred to as a `searcher receiver`, or simply `searcher`, to track the frequency of the pilot and obtain synchronization with the pilot timing. Several techniques and devices have been used to provide this searcher function. One such technique is disclosed in U.S. Pat. No. 5,109,390 entitled "Diversity Receiver In A CDMA Cellular Telephone System," issued Apr. 28, 1992, which is assigned to the assignee of the present invention, and incorporated herein by reference.
To actually acquire a pilot signal, the searcher receiver despreads pilot signals using a correlator with system PN codes generated in the subscriber unit using local reference timing. After this despreading, signal amplitudes are measured for received signal chips and accumulated over a preselected interval of time. This provides a coherent sum of chip amplitudes. A number of such accumulated values are squared and then further summed (both I and Q) to produce a non-coherent sum which is compared to one or more predetermined threshold levels. Non-coherent sums exceeding desired thresholds generally indicate that appropriate pilot signal timing has been selected.
One problem that occurs during the accumulation process is that certain Walsh functions useful for generating individual orthogonal communication channels for subscriber units, tend to interfere both constructively and destructively with each other under certain conditions. For example, when using a set of 128 Walsh functions of length 128, functions 0 and 64 tend to cause constructive or destructive interference when signal amplitudes are accumulated over intervals that are less than or equal to one-half of the Walsh function length. This interference associated with short accumulations tends to increase the likelihood of false alarms (deciding the current timing hypothesis is correct when it's not) and misses (deciding the current timing hypothesis is wrong when it's actually correct).
This interference is exhibited for more Walsh functions or communication channels the shorter the accumulation interval becomes, such as one-half versus one-quarter versus one-eighth of a function length. However, it is unacceptable to simply increase the length of the integration time to compensate. This is complicated by the fact that future communication systems may use multiple PN codes and other features or changes in signal parameters that also require search space. That is, the searcher may be searching across other parameters besides Doppler frequency and PN timing, and each parameter increases the length of a search by a factor of two or more. Therefore, time is at a premium for each search factor or parameter.
Therefore, it is desirable to have a method and apparatus for signal acquisition that provides faster and more reliable signal acquisition when using integration or accumulation windows that are less than or equal to one-half of the channel forming function length. It is also desirable to make more efficient use of signal energy, or more appropriately distribute higher use or power signals among the various orthogonal channels in a communication system to provide improved energy capture and signal acquisition.